This invention relates to methods and apparatus for amplifying signals. More particularly, this invention relates to amplifying signals using an arrangement of multiple amplifiers.
It is known to use highly non-linear power amplifiers in combination to create a linear amplifier. The aim is usually to create a very high efficiency linear amplifier, based on the high efficiency of the non-linear power amplifiers (e.g. class C, D or E).
One such technique is called LINC (linear amplification using non-linear components). This technique converts the desired amplitude and phase modulated signal into two constant-envelope phase-modulated signals. The key property of these signals is that when they are summed, the result is the desired amplitude and phase modulated carrier.
A major disadvantage of the technique results from this summation process in that the wanted aspects of the signals add, but the unwanted aspects subtract and this part of the signal energy is wasted by being dissipated in the load attached to the summing device (e.g. a hybrid combiner). Thus, even with perfect (100% efficient) power amplifiers, the resulting LINC system efficiency will only be 50% (for a standard two-tone test with equal tone amplitudes). With practical amplifiers (e.g. class-C operating at 60% efficiency), this figure can reduce to 30% overall efficiency, which, although better than an equivalent class-A amplifier, is still not particularly good. With high peak-to-mean signals (e.g. CDMA), this figure reduces still further.
Another known technique is LIST (linear amplification using sampling techniques). In this case, delta-modulated signals are amplified and combined in quadrature to produce the desired output signal. Cancellation at the output is required to remove image signals and not unwanted adjacent channel energy. The resulting effect on efficiency is, however, the same.
It is an object of the present invention to provide an amplification technique of improved linearity and/or efficiency.
According to a first aspect, the invention provides apparatus for amplifying an input signal to produce an output signal, the apparatus comprising splitting means for providing the input signal to each of a plurality of selectable amplifiers via a respective switch, control means for controlling the operation of the switches according to the envelope of the input signal, and a combiner for combining the selectable amplifier outputs.
According to a second aspect, the invention provides a method of amplifying an input signal to produce an output signal, the method comprising providing the input signal to each of a plurality of selectable amplifiers via a respective switch, controlling the operation of the switches according to the envelope of the input signal, and combining the selectable amplifier outputs.
In this amplification scheme, the switches can be used to bring the selectable amplifiers into operation as necessary to achieve a desired gain. This scheme also permits the selectable amplifiers to be designed for optimum efficiency at the gain level corresponding to the contribution which they can make to the overall gain.
Preferably, at least one of the selectable amplifiers has a different output rating to the other(s). This allows greater flexibility, for a given number of selectable amplifiers, to be achieved in terms of the overall system gain. The output ratings of the selectable amplifiers may be arranged such that each is different and such that the output ratings of the selectable amplifiers comprise a sequence wherein each successive output rating is twice the preceding one. This affords yet greater flexibility in terms of overall gain for a given number of selectable amplifiers, and provides that the selectable amplifiers can be operated by a digital, binary input.
In the preferred embodiment, the control means drives the switches by means of such a digital signal. The digital signal may be either the digitised, detected envelope of the input signal or it may be the input signal itself, where the input signal is a digital, baseband signal destined for modulation. Each bit of the digitised signal may be used to control a respective switch in the operation of a corresponding selectable amplifier. A digital feedback signal derived from the output of the amplifying arrangement may be used to adapt the operation of the switches to counter errors observed in the overall output.
The selectable amplifiers can be supplemented with an additional amplifying means arranged to also receive the input signal from the splitting means and provide an amplified signal to the combiner. The additional amplifying means may be included to reduce the complexity of the scheme by providing, in addition, sufficient selectable amplifiers to handle signals above a certain envelope threshold only. This means that the amplification is handled solely by the additional amplifying means when the envelope is below the threshold rather than by several selectable amplifiers. The input signal to the additional amplifying means may be modulated under the control of the control means (especially where the input signal to the splitting means is subject to amplitude limitation by a suitable device). Alternatively, the input to the additional amplifying means may be clipped by an appropriate device so that the additional amplifying means only operates on signals having an envelope up to the threshold value.
In a preferred embodiment, the input signal to the splitting means is amplitude-limited so that the version of the input signal supplied to the splitting means has a substantially constant amplitude. Amplitude variations may then be reintroduced to the scheme by using the control means to select certain of the selectable amplifiers (or modulate the input to the additional amplifying means, if included).